thickbox.css needs display:none

Description

In thickbox.css display:none was replaced by visibility:hidden (line 52). The thickbox will not be displayed if there is no property 'display', because thickbox checks the property 'display' (line 208 in thickbox.js).
If you use e.g. tb_show in a javascript, it will not show the thickbox.

This was changed because the uploader had problems initializing in a hidden iframe in some browsers. Seems 18796.2.patch is the proper solution here and the overlapping can probably be solved with css.